"..on the banks of a very picturesque mountain stream that pours out its crystalline water in four or five waterfalls into the Dussel brook.. .Oh, in this cave, at this crystal flood, I often felt myself so well! Sensations frequently welled up in my bosom at this blessed place that ennoble the soul and make pour out joyful tombs; give the heart impressions that neither greatness or honor can steal from us. An indomitable longing came to me, to learn more and more about these enchanting shades of beautiful and holy nature, and to transfer them on the canvas with my brush."

“In my opinion the aim of the painter is similar with that of the poet, insofar that both want to affect the feelings of the viewer or reader. As soon as their scenes.. ..are lacking the mark of nature, of truth, than both will fail to realize it. The Dutch painter feels - as well as the Germans do - the influence of sublime nature, but the Dutch painter first wants to be acquainted with 'plain truth', to combine it afterwards with the poetic..”

“I really believe that the general, happy mood of the people here is largely caused by nature. At least I am experiencing that in places like these people are much more natural than in regions where nature offers them little or nothing to subtract their heart for some time from the hypocrisy of the world, and to taste a not-deceitful delight.”

“official website of the museum Koekkoek-Haus, Kleve, in English language”

“Immediately and definitively I determine the effect of sunlight, day and shadow, without being concerned with any detail. This enable me to see in the started painting the whole of it, which my mind already saw before I begun to work - I see it appearing on the panel or canvas rather soon and can consider in this way the harmony of the composed objects and colors.”

“Fortunately however, is that school where Mother Nature is placed in the foreground, and where only she is consulted to representate 'truth' on the canvas or panel. Only he knows the secrets of the manifold diversity of nature. His painting is a faithful copy of nature - which is the highest praise for a painter..”

“Beforehand I don't make any drawing of the object or objects which I want to paint on the canvas or panel.. ..but I start directly to situate the designed plan on the canvas - After having thoroughly sketched and thought over my composition, especially the arrangement of light and dark, I start to paint it broadly with oil-paint and try as much as possible to achieve the hue or the colouring, in which I want to see my landscape.. ..when it is definitely completed.”